1. Field of the Invention
The present invention specifically relates to, of light irradiation devices which irradiate light on a hologram recording medium, a light irradiation device including a mechanism for adjusting distance between an objective lens and a relay lens, and also relates to a control method for controlling the distance between the objective lens and the relay lens so as to be constant.
2. Description of the Related Art
The hologram recording/reproducing method for executing recording of data using a hologram format has been in practical use, as disclosed in Japanese Unexamined Patent Application Publication No. 2007-79438 for example. With the hologram recording/reproducing method, at the time of recording, signal light to which spatial light intensity modulation (intensity modulation) according to data to be recorded is applied, and reference light to which a predetermined light intensity pattern is applied are generated, and these are irradiated on a hologram recording medium, thereby forming a hologram on a recording medium to execute recording of data.
Also, at the time of reproducing, the reference light is irradiated on a recording medium. Thus, the same reference light as at the time of recording (having the same intensity pattern as at the time of recording) is irradiated on the hologram formed according to irradiation of signal light and reference light at the time of recording, thereby obtaining diffracted light according to the recording signal light component. That is to say, the reproduced image corresponding to data thus recorded (reproduced light) is obtained. The reproduced light thus obtained is detected by an image sensor, for example, such as a CCD (Charge Coupled Device) sensor, a CMOS (Complementary Metal Oxide Semiconductor) sensor, or the like, thereby reproducing recorded information.
Also, as such a hologram recording/reproducing method, so-called coaxial method has been in use wherein reference light and signal light are disposed on the same optical axis, and these are irradiated on a hologram recording medium via a common objective lens.
Here, with the hologram recording/reproducing method, the signal light is made up of data of zeroes and ones being arrayed two-dimensionally by an on/off pattern of light being applied as a light intensity pattern. That is to say, the signal light handles multiple bits worth of information of recorded data. With the hologram recording/reproducing method, the increments of multiple bits worth of data that can be arrayed within the signal light are taken as the minimum increments of recording/reproducing. A hologram to be recorded with one-time interference between the signal light and reference light is referred to as “hologram page” in that multiple data bits are included such as described above.
With the hologram recording/reproducing system, data is sequentially recorded in increments of such hologram pages. Currently, with the system using the above coaxial method in particular, an arrangement can be conceived to execute recoding of data in increments of such hologram pages while rotationally driving a disc-shaped recording medium such as an optical disc system such as CD (Compact Disc) or DVD (Digital Versatile Disc) according to the related art, or the like.
In this case, tracks are formed spirally or concentrically as to a disc-shaped hologram recording medium beforehand, and formation of a hologram according to irradiation of signal light/reference light is executed sequentially while tracing the tracks, thereby forming a hologram page along the tracks.
In the case of using the technique for thus forming a hologram page at a position along with the tracks, control of recording/reproducing position has to be executed, such as tracking servo for tracing a beam spot on the tracks, or access control as to a predetermined address, or the like.
Currently, it has also been conceived to irradiate a dedicated laser beam separately at the time of executing such control of a recording/reproducing position. That is to say, this is a technique for irradiating a laser beam used for recording/reproducing a hologram (laser beam for irradiating signal light/reference light; laser beam for recording/reproducing), and a laser beam used for controlling the recording/reproducing position of a hologram (laser beam for position control) separately.
In order to handle such a technique for irradiating a laser beam used for position control separately, the hologram recording medium is configured so as to have a cross-sectional configuration such as shown in FIG. 22, for example.
With the hologram recording medium HM shown in FIG. 22, a recording layer L1 where recording of a hologram is executed, and a position control information recording layer where address information for position control, and the like are recorded with an uneven cross-sectional configuration on the substrate L6, are formed separately.
Specifically, with the hologram recording medium HM, a cover layer L1, a recording layer L2, a reflective film L3, an intermediate layer L4, a reflective film L5, and a substrate L6 are formed. The reflective film L3 which is formed on the lower layer of the recording layer L2 is provided, on which reference light according to the laser beam for recording/reproducing is irradiated at the time of reproducing, and when the reproduced image corresponding to the hologram recorded on the recording layer L2 is obtained, to return this to the device side as reflected light.
Also, with the substrate L6, a track for guiding the recording/reproducing position of a hologram on the recording layer L2 is formed spirally or concentrically. For example, the track is formed by recording of information such as address information by a pit row, or the like being executed.
The reflective film L5 formed on the upper layer of the substrate L6 is provided to obtain reflected light regarding the information recorded on the substrate L6. Note that the intermediate layer L4 is provided as an adhesive material, for example, such as a resin or the like.
Here, in order to execute suitable position control based on the reflected light of the laser beam for position control regarding the hologram recording medium HM having the cross-sectional configuration such as described above, the laser beam for position control has to reach as far as a reflective film L5 to which an uneven cross-sectional shape is given. That is to say, from this point of view, the laser beam for position control has to transmit the reflective film L3 formed on an upper layer than the reflective film L5.
On the other hand, in order to return the reproduced image corresponding to the hologram recorded on the recording layer L2 to the device side as reflected light, the reflective film L3 has to reflect the laser beam for recording/reproducing.
These points are taken into consideration, and accordingly, a laser beam having a different wavelength from the laser beam for recording/reproducing a hologram is used as the laser beam for position control. For example, a blue-violet laser beam with a wavelength λ=405 nm or so is used as the laser beam for recording/reproducing a hologram, and on the other hand, for example, a red laser beam with a wavelength λ=650 nm or so is used as the laser beam for position control.
Thereupon, a reflective file having wavelength selectivity that the blue-violet laser beam for recording/reproducing is reflected, the red laser beam for position control is transmitted is used as the reflective film L3 formed between the recording layer L2 and the reflective film L5 where recording of position control information has been executed.
According to such a configuration, at the time of recording/reproducing, the laser beam for position control reaches the reflective film L5, and reflected light information for position control is suitably detected, and also the reproduced image of the hologram recorded on the recording layer L2 is suitably detected on the device side.
FIG. 23 is a diagram illustrating the configuration of a recording/reproducing device serving as an example of the related art for executing recording/reproducing as to the hologram recording medium HM having the above configuration (principally regarding the optical system alone) in a simple manner.
First, with the recording/reproducing device, an optical system for irradiating signal light and reference light for recording/reproducing of a hologram is provided with a first laser 1, a collimation lens 2, a polarized beam splitter 3, an SLM 4, a polarized beam splitter 5, a relay lens 6, a relay lens 7, a dichroic mirror 8, a partial diffraction element 9, a quarter-wave plate 10, an objective lens 102, and an image sensor 13.
The first laser 1 outputs, for example, the above blue-violet laser beam with a wavelength λ=405 nm or so as the laser beam for recording/reproducing a hologram. The laser beam emitted from the first laser 1 is input to the polarized beam splitter 3 via the collimation lens 2.
The polarized beam splitter 3 transmits, of linear polarized light components orthogonal to each input laser beam, one of the linear polarized light components, and reflects the other linear polarized light component. For example, in this case, the polarized beam splitter 3 is configured so as to transmit a p-polarized light component, and reflect an s-polarized light component. Accordingly, with the laser beam input to the polarized beam splitter 3, the s-polarized light component alone is reflected so as to be guided to the SLM 4.
The SLM 4 is configured so as to include a reflection-type liquid crystal element serving as an FLC (Ferroelectric Liquid Crystal) for example, and is configured so as to control the polarization direction as to incident light in increments of pixels.
This SLM 4 executes spatial light modulation for each pixel, so as to change the polarization direction of incident light 90 degrees, or so as not to change the polarization direction of incident light, according to the driving signal from the modulation control unit 101 in the drawing. Specifically, the SLM 4 is configured so as to execute polarization direction control in increments of pixels according to the driving signal so that angular change in the polarization direction is 90 degrees regarding a pixel of which the driving signal is turned on, and angular change in the polarization direction is 0 degree regarding a pixel of which the driving signal is turned off.
As shown in the drawing, the emission light from the SLM 4 (light reflected at the SLM 4) is input to the polarized beam splitter 3 again. Here, the recording/reproducing device shown in FIG. 23 executes polarization direction control in increments of pixels by the SLM 4, and spatial light intensity modulation (light intensity modulation, or simply referred to as “intensity modulation”) in increments of pixels by taking advantage of the selective transmission/reflection properties of the polarized beam splitter 3 according to the polarization direction of incident light.
FIGS. 24A and 24B illustrate the imaginary of intensity modulation realized by a combination between such an SLM 4 and a polarized beam splitter 3. FIG. 24A schematically illustrates the light of a pixel of which the driving signal is on, and FIG. 24B schematically illustrates the light of a pixel of which the driving signal is off, respectively.
As also described above, the polarized beam splitter 3 transmits p-polarized light, and reflects s-polarized light, and consequently, s-polarized light is input to the SLM 4.
According to this premise, the light of a pixel of which the polarization direction is changed 90 degrees by the SLM 4 (light of a pixel of which driving signal is on) is input to the polarized beam splitter 3 using the p-polarized light. Thus, the light of a pixel of which the driving signal is on at the SLM 4 transmits the polarized beam splitter 3, and is guided to the hologram recording medium HM side (FIG. 24A).
On the other hand, the light of a pixel of which the driving signal is off, and the polarization direction is not changed is input to the polarized beam splitter 3 using the s-polarized light. That is to say, the light of a pixel of which the driving signal is off at the SLM 4 is reflected at the polarized beam splitter 3, and is guided to the hologram recording medium HM side (FIG. 24B).
Thus, an intensity modulating unit configured to subject to light intensity modulation in increments of pixels is made up of a combination between the polarization direction control-type SLM 4 and the polarized beam splitter 3. According to such an intensity modulating unit, signal light and reference light are generated at the time of recording, and reference light is generated at the time of reproducing.
The laser beam for recording/reproducing subjected to spatial light modulation by the intensity modulating unit is input to the polarized beam splitter 5. This polarized beam splitter 5 is also configured so as to transmit p-polarized light and reflect s-polarized light, and accordingly, the laser beam emitted from the intensity modulating unit (light transmitted through the polarized beam splitter 3) transmits the polarized beam splitter 5.
The laser beam transmitted through the polarized beam splitter 5 is input to a relay lens system where the relay lens 6 and the relay lens 7 are disposed in this order. As shown in the drawing, the light flux of the laser beams transmitted through the polarized beam splitter 5 condenses in a predetermined focal position by the relay lens 6, and the laser beam flux which is diffusion light after condensing is converted into parallel beams by the relay lens 7.
The laser beam passed through the relay lens system is input to the dichroic mirror 8. The dichroic mirror 8 is configured so as to selectively reflect light according to a predetermined wavelength band. Specifically, this case is configured so as to selectively reflect the light of the wavelength band of the laser beam for recording/reproducing according to a wavelength λ of 405 nm or so. Accordingly, the laser beam for recording/reproducing input via the relay lens system is reflected at the dichroic mirror 8.
The laser beam for recording/reproducing reflected at the dichroic mirror 8 is input to the objective lens 102 via the partial diffraction element 9 and the quarter-wave plate 10. The partial diffraction element 9 and the quarter-wave plate 10 are provided to prevent reference light, reflected at the hologram recording medium HM at the time or reproducing (reflected reference light), from being guided to the image sensor 13 and becoming noise in the reproduced light. Note that the suppression operation of reflected reference light by the partial diffraction element 9 and the quarter-wave plate 10 will be described later.
The objective lens 102 is held movably in the focus direction (direction toward/away from the hologram recording medium HM) by a focus actuator 12 shown in the drawing. The later-described position control unit 19 controls the driving operation of the objective lens 102 by the focus actuator 12, and accordingly, the focus servo control of the laser beam is executed.
Note that while omitted from the drawings, control of the tracking direction of the laser beam (the radial direction of the hologram recording medium HM) can be executed, for example, by controlling a tracking driving unit configured to drive the whole of the optical system in the tracking direction, or the like.
The laser beam for recording/reproducing is irradiated on the hologram recording medium HM so as to condense by the objective lens 102. Here, as also described above, at the time of recording, signal light and reference light are generated by intensity modulation by the intensity modulating unit (SLM 4 and polarized beam splitter 3), and the signal light and reference light are irradiated on the hologram recording medium 100 using the routes described above. Thus, a hologram to which recorded data is reflected using an interference pattern between the signal light and the reference light is formed on the recording layer L2, and data recording is realized.
Also, at the time of reproducing, the reference light alone is generated by the intensity modulating unit, and is irradiated on the hologram recording medium HM using the above route. The reference light is thus irradiated, and accordingly, the reproduced image corresponding to the hologram formed on the recording layer L2 is obtained as the reflected light from the reflective film L3. This reproduced image is returned to the device side via the objective lens 102.
Here, the reference light irradiated on the hologram recording medium HM at the time of reproducing (referred to as “outbound reference light”) is input to the partial diffraction element 9 using the p-polarized light according to the operation of the previous intensity modulating unit. As also described later, the partial diffraction element 9 is configured so as to transmit all outbound light, and accordingly, the outbound reference light using the p-polarized light is passed through the quarter-wave plate 10. Thus, the outbound reference light using the p-polarized light via the quarter-wave plate 10 is converted into circularly polarized light in a predetermined rotational direction, and is irradiated on the hologram recording medium HM.
The reference light irradiated on the hologram recording medium HM is reflected at the reflective film L3, and is guided to the objective lens 102 as reflected reference light (inbound reference light). At this time, the circularly polarized light rotational direction of the homeward reference light is converted to the rotational direction opposite to the predetermined rotational direction due to reflection off of the reflective film L3, and accordingly, the homeward reference light is converted into s-polarized light by passing through the quarter-wave plate 10.
Now, based on transition of such a polarized state, the suppression operation of the reflected reference light by the partial diffraction element 9 and the quarter-wave plate 10 will be described.
The partial diffraction element 9 is configured of a polarized light selecting diffraction element having the selective diffraction property corresponding to the polarized state of linear polarized light (one of the linear polarized light component is diffracted, and the other linear polarized light component is transmitted) being formed with a region where reference light is input (region except for the center portion), for example, such as a liquid crystal element or the like. Specifically, in this case, the polarized light selecting diffraction element included in the partial diffraction element 9 is configured so as to transmit p-polarized light and diffract s-polarized light. Thus, the outbound reference light transmits the partial diffraction element 9, and the reference light alone of the inbound path is diffracted (suppressed) at the partial diffraction element 9.
As a result thereof, prevention of a situation where the reflected reference light which is inbound light is detected as a noise component as to the reproduced image, and the S/N ratio deteriorates is achieved.
Note that a region where the signal light is input of the partial diffraction element 9 (region where the reproduced image is input) is configured of, for example, a transparent material, or a hole portion so as to transmit both of the outbound light and the inbound light. Thus, the signal light at the time of recording, and the reproduced image at the time of reproducing are arranged so as to transmit the partial diffraction element 9.
Now, as can be understood from the above description, with the hologram recording/reproducing system, reference light is irradiated on the recorded hologram, and a diffraction phenomenon is taken advantage of so as to obtain a reproduced image, but the diffraction efficiency at this time is generally less than several percentages through 1 percentage. Thus, the reference light to be returned to the device side as reflected light such as described above has great intensity as to the reproduced image. That is to say, the reference light which is the reflected light becomes a noise component not negligible for detection of a reproduced image. Accordingly, suppression of the reflected reference light is achieved by the partial diffraction element 9 and the quarter-wave plate 10, and thus, great improvement in the S/N ratio is achieved.
The reproduced image obtained at the time of reproducing such as described above transmits the partial diffraction element 9. The reproduced image transmitted through the partial diffraction element 9 is irradiated at the dichroic mirror 8, and is then input to the polarized beam splitter 5 via the above-described relay lens system (relay lens 7, relay lens 6, in this order). As can be understood from the above description, the reflected light from the hologram recording medium HM is converted into s-polarized light via the quarter-wave plate 10, and accordingly, the reproduced image thus input to the polarized beam splitter 5 is reflected at the polarized beam splitter 5, and is input to the image sensor 13.
Thus, at the time of reproducing, the reproduced image from the hologram recording medium HM is detected by the image sensor 13, and accordingly, reproducing of data by a data reproducing unit 21 in the drawing is executed.
Also, with the recording/reproducing device shown in FIG. 23, there is provided an optical system for executing irradiation of a laser beam for position control, and detection of reflected light of the laser beam for position control. Specifically, this optical system is made up of a second laser 14, a collimation lens 15, a polarized beam splitter 16, a condenser lens 17, and a photodetector (PD) 18 shown in FIG. 23.
The second laser 14 outputs the above red laser beam with a wavelength λ of 650 nm or so the laser beam for position control. The emission light from the second laser 14 is input to the dichroic mirror 8 via the collimation lens 15, and the polarized beam splitter 16 in this order. Here, the polarized beam splitter 16 is also configured so as to transmit p-polarize light, and reflect s-polarized light.
As also described above, the dichroic mirror 8 is configured so as to selectively reflect a laser beam for recording/reproducing (405 nm in this case), and accordingly, transmits the laser beam for position control from the second laser 14.
The laser beam for position control transmitted through the dichroic mirror 8 is irradiated, in the same way as the laser beam for recording/reproducing, on the hologram recording medium HM via the partial diffraction element 9, quarter-wave plate 10, and objective lens 102 in this order.
Note that the dichroic mirror 8 is provided, and accordingly, the laser beam for position control and the laser beam for recording/reproducing are synthesized on the same optical axis, and also this synthesized light is irradiated on the hologram recording medium HM via the common objective lens 102. That is to say, thus, an arrangement is made wherein the beam spot of the laser beam for position control and the beam spot of the laser beam for recording/reproducing are formed on the same position in a direction within the recording surface, and consequently, position control operation based on the laser beam for position control such as described below is executed, and accordingly, the recording/reproducing position of the hologram is controlled so as to position on a track.
Also, with regard to focus direction, according to position control operation as described below (focus servo control), the focal position of the laser beam for position control is controlled so as to position on the reflective film L5 of the hologram recording medium HM (see FIG. 22).
At this time, with the recording/reproducing device, adjustment is executed so that the focal position of the laser beam for position control, and the focal position of the laser beam for recording/reproducing are separated by predetermined distance. Specifically, in this case, the laser beam for recording/reproducing condenses in the reflective film L3 immediately below the recording layer L2, and accordingly, adjustment is executed so that the focal position of the laser beam for recording/reproducing is disposed in the front side by distance from the reflective film L5 surface to the reflective film L3 surface as to the focal position of the laser beam for position control (see FIG. 22).
Thus, an arrangement is made wherein as focus servo is executed with the focal position of the laser beam for position control as on the reflective film L5, the focal position of the laser beam for recording/reproducing is automatically set to above the reflective film L3.
In FIG. 23, in response to the laser beam for position control being irradiated on the hologram recording medium HM, the reflected light corresponding to the recorded information on the reflective film L5 is obtained. This reflected light is input to the polarized beam splitter 16 via the objective lens 102, quarter-wave plate 10, partial diffraction element 9, and dichroic mirror 8 in this order. Thus, the polarized beam splitter 16 reflects the reflected light of the laser beam for position control input thereto via the dichroic mirror 8 (the laser beam for position control reflected at the hologram recording medium HM is also converted into s-polarized light by operation of the quarter-wave plate 10). The reflected light of the laser beam for position control reflected at the polarized beam splitter 16 is irradiated on the detection surface of the photodetector 18 via the condenser lens 17 so as to condense.
The photodetector 18 receives the reflected light of the laser beam for position control irradiated such as described above, converts this into an electrical signal, and supplies this to the position control unit 19.
The position control unit 19 is configured so as to include a matrix circuit for generating various types of signals used for position control such as a reproducing signal (RF signal) regarding a pit row formed on the reflective film L5 by matrix computation, a tracking error signal, a focus error signal, and the like, a computing circuit for generating a servo signal, and a driving control unit for driving and controlling each used unit such as the focus actuator 12, the above tracking driving unit, and the like.
While omitted from the drawings, with the recording/reproducing device, there are provided an address detecting circuit and a clock generating circuit which are used for executing detection of address information and generation of clock based on the above reproducing signal.
The position control unit 19 controls the tracking driving unit based on the address information and the tracking error signal, thereby controlling the beam spot position of the laser beam for position control. According to control of such a beam spot position, the beam spot position of the laser beam for recording/reproducing can be moved to a predetermined address, or can be followed on the tracks (tracking servo control), or the like. That is to say, thus, control regarding the recording/reproducing position of a hologram is executed.
Also, the position control unit 19 controls the driving operation of the objective lens 102 in the focus direction by the focus actuator 12 based on the focus error signal, thereby executing focus servo control for following the focal position of the laser beam for position control on the reflective film L5. As described above, focus servo control regarding such a laser beam for position control is executed, and accordingly, the focal position of the laser beam for recording/reproducing follows on the reflective film L3.
Here, focus servo control such as described above is executed, and accordingly, along with the position in the focus direction of the objective lens 102 changing so as to follow the plane shaking of the hologram recording medium HM or the like, distance between the relay lens 7 and the objective lens 102 changes successively.
With a hologram recording/reproducing system, upon distance between the relay lens 7 and the objective lens 102 being thus changed by focus servo control, recording/reproducing a hologram is not readily executed in that 1) reference light is not readily irradiated with the same conditions at the time of recording/reproducing, and 2) blurring occurs on a reproduced image, a fact which is widely recognized.
This point will be described with reference to FIGS. 25 and 26. FIG. 25 illustrates the behavior of light with the optical system regarding light emitted from each pixel of the SLM 4. Note that, in FIG. 25, with regard to signal light to be generated along with the spatial light modulation of the SLM 4, only three pixels worth of light beam thereof are illustrated, and with regard to reference light, only two pixels worth of light beam are illustrated.
Also, in FIG. 25, of the configuration of the whole optical system, only the SLM 4, relay lenses 6 and 7, and objective lens 102 are extracted and illustrated. Also, in this drawing, the hologram recording medium HM is also illustrated together therewith.
Also, the flat surface Spbs in the drawing represents the reflective surface of the polarized beam splitter 5, and also the flat surface Sdim represents the reflective surface of the dichroic mirror 8. Also, the Fourier surface (frequency flat surface) SF in the drawing is the focal point surface of the relay lens 6, and becomes a surface coupled to the focal point surface of the objective lens 102. Also, the real image surface SR in the drawing is the object surface of the objective lens 102, and is coupled to the modulation surface (image generating surface) of the SLM 4. The light reception surface of the image sensor 13 is coupled to the real image surface SR.
Here, let us say that the positions of the objective lens 102 and the relay lens 7 shown in FIG. 25 are each an ideal position assumed at the time of optical system design. Note that, in the case of the recording/reproducing device according to the related art shown in FIG. 23, the position where each light beam passes through is the same regarding outbound and inbound, and accordingly, one drawing is used to show both.
As shown in FIG. 25, the light beam emitted from each pixel of the SLM 4 is input to the relay lens 6 via the flat surface Spbs (polarized beam splitter 5) in the state of diffusion light. At this time, the emitted light beam from each pixel is in a state in which each optical axis is in parallel.
The light beam of each pixel input to the relay lens 6 is converted from the diffusion light to parallel light such as shown in the drawing, and also the optical axis of each light beam except from the light beam on the laser beam axis (the optical axis of the whole laser beam flux) is bent on the laser beam axis side. Thus, with the Fourier surface SF, each light beam condenses on the laser beam axis in a parallel light state.
Each light beam condensing on the laser beam axis of the Fourier surface SF such as described above is input to the relay lens 7, but at this time, each light beam (except for the light beam of the center pixel including the laser beam axis) emitted from the relay lens 6 intersects the laser beam axis on the Fourier surface SF. Thus, the relationship of the input/output position of each light beam between the relay lens 6 and the relay lens 7 assumes an axial symmetrical relationship with the laser beam axis as the center.
Each light beam is converted into converged light by passing through the relay lens 7 such as shown in the drawing, and also the optical axis of each light beam becomes parallel. Each light beam passed through the relay lens 7 is reflected at the flat surface Sdim (dichroic mirror 8), and condenses in the corresponding position on the real image surface SR. At this time, each light beam via the relay lens 7 is in a state in which each optical axis is in parallel such as shown in the above, and accordingly, the condensing position of each light beam is not overlapped, and becomes a different position, on the real image surface SR.
Each light beam condensing on the real image surface SR such as shown in the drawing is input to the objective lens 102 in a diffusion light state (the optical axis of each light beam is in parallel at this time). Each light beam input to the objective lens 102 is in a parallel state such as shown in the drawing. With the reflective surface (reflective film L3 surface) serving as the condensing surface of the objective lens 102 in the drawing, each light beam in such a parallel light state condenses on the laser beam axis. Note that, as can be understood from this, the focal point surface of the objective lens 102 and the above Fourier surface SF have coupling relationship.
Here, FIG. 25 illustrates each light beam of the reproduced light reflected at the flat surface Spbs and guided to the image sensor 13 (each light beam of the reproduced image obtained within the signal light area A2 at the time of reproducing), but the reason why the reproduced light alone is guided to the image sensor 13 such as shown in the drawing is because the reflected reference light is suppressed by the partial diffraction element 9 (and the quarter-wave plate 10) described above.
Note that each light beam of the reproduced light reaches the flat surface Spbs passing through the same positions as with each light beam of the signal light in the drawing, and is reflected at this flat surface Spbs and guided to the image sensor 13.
At this time, each light beam of the reproduced light emitted to the flat surface Spbs side from the relay lens 6 is in a converged light state and also in a state where each optical axis is in parallel, and condenses in a separate position on the detection surface of the image sensor 13. Thus, the same image as the reproduced image on the real image surface SR is obtained on the detection surface of the image sensor 13.
FIG. 26 illustrates the situation of each light beam of signal light and reference light in a state where the reflective surface and the objective lens 102 are in an ideal position ((a) in FIG. 26), and the situation of each light beam at the time of the objective lens 102 being driven following the displacement from the ideal position of the reflective surface ((b) in FIG. 26) in a comparative manner.
Note that, in these drawings, each light beam which is outbound light on the real image surface SR shown in FIG. 25 and thereafter is indicated with a solid line. A dashed line in the drawing indicates the situation of each light beam reflected at the reflective surface (just the reference light beams in the drawings, for the sake of convenience).
First, such as shown in (a) in FIG. 26, in a state in which the reflective surface and the objective lens 102 are each disposed in an ideal position, such as described in FIG. 25, each light beam emitted from the objective lens 102 is in a parallel light state, and accordingly, each light beam reflected at the reflective surface is also returned to the objective lens 102 side in a parallel light state.
Thus, such as described above in FIG. 25, each light beam of the reproduced image is obtained in the same light beam region as each light beam of the signal light at the time of recording.
Let us say that from this state shown in (a) in FIG. 26, the reflective surface moves by ΔZ in a direction away from the objective lens 102 due to plane shaking or the like, and along therewith, the objective lens 102 is driven by focus servo control toward the reflective surface by ΔZ.
In response to this, distance from the real image surface SR to the objective lens 102 is separated by ΔZ such as shown in (b) in FIG. 26, and accordingly, the width of each light beam input to the objective lens 102 is expanded.
Thus, the width of each light beam input to the objective lens 102 is expanded from the ideal state shown in (a) in FIG. 26, and accordingly, each light beam emitted from the objective lens 102 is not in a parallel light state but in a converged light state such as shown in the drawing.
Here, (b) in FIG. 26 illustrates a case where the reflective surface moves in a direction away from the objective lens 102 as an example, but conversely, in the case that the reflective surface moves in a direction toward the objective lens 102, the objective lens 102 is driven so as to move to the light source side, and the width of each light beam input to the objective lens 102 becomes thinner than in the case of (a) in FIG. 26. That is to say, in this case, conversely, each light beam emitted from the objective lens 102 is in a diffusion light state.
As can be understood from the above description, in response to the objective lens 102 being driven so as to follow the operation of the reflective surface by focus servo, the state of each light beam of the signal light and reference light emitted from the objective lens 102 differs successively. Therefore, reference light is not readily irradiated with the same conditions at the time of recording/at the time of reproducing, and accordingly, a hologram is not readily suitably recorded.
Also, in response to each light beam emitted from the objective lens 102 being not in a parallel light state such as described above, the focal position of each light beam which is inbound light is not identical to the real image surface SR, and accordingly, at the time of reproducing, blurring also occurs on an image received at the image sensor 13.
That is to say, such as shown in (b) in FIG. 26, in the case that the reflective surface moves in a direction away from the objective lens 102, and the emission light from the objective lens 102 becomes converged light, with the inbound path, the width of each light beam input to the objective lens 102 from the reflective surface is narrower than the case of (a) in FIG. 26, and consequently, each light beam which is inbound light emitted from the objective lens 102 focuses on the front side of the real image surface SR.
Also, as can be understood from this, conversely, in the case that the reflective surface moves toward the objective lens 102, each light beam which is inbound light emitted from the objective lens 102 focuses on a deeper side than the real image surface SR.
As also described above, the real image surface SR (the real image surface of the outbound path) and the light reception surface of the image sensor 13 have coupling relationship, and accordingly, the same focal position deviation also occurs on the image sensor 13, and according thereto, blurring of the reproduced image occurs.
In order to solve such a problem, the recording/reproducing device according to the related art has a configuration for maintaining distance between the objective lens 102 which displaces successively by focus servo, and the relay lens 7 so as to be constant. Specifically, this is made up of a relay lens driving unit 103, a position sensor 104, a position sensor 105, and a relay lens position control unit 106, shown in FIG. 23.
With the recording/reproducing device according to the related art, each of the positions of the objective lens 102 and the relay lens 7 is detected by the position sensors 104 and 105, and control is executed so that the position from the ideal position of each of the objective lens 102 and the relay lens 7 (movement distance) becomes the same.
Specifically, the relay lens position control unit 106 obtains the motion amount of the objective lens 102 (movement amount ΔZ: having ± polarity with the ideal position as a reference) from the position information of the objective lens 102 detected by the position sensor 104, and controls the relay lens driving unit 103 so that the relay lens 7 is moved until the motion amount (movement amount) of the relay lens 7 obtained from the position information detected by the position sensor 105 becomes the above movement amount ΔZ. Thus, the distance between the objective lens 102 and the relay lens 7 can be maintained constant.
In the case that the distance between the objective lens 102 and the relay lens 7 is maintained constant, distance from the real image surface SR serving as the condensing point of each light beam emitted from the relay lens 7 to the objective lens 102 can be maintained constant. That is to say, the distance between the real image surface SR and the objective lens 102 can be set to the distance shown in (a) in FIG. 26 so as to be constant, and the emission light from the objective lens 102 can become constant parallel light. As a result thereof, even in the event that the objective lens 102 displaces according to focus servo, recording/reproducing can suitably be executed.